The present invention relates to a process of preparing a composite wire preferably employed for a lead wire of such a sensor as a temperature sensor and an oxygen sensor.
A wire composed of platinum or its alloy has been employed for the above sensors. However, the recrystallization temperature of the platinum or its alloy is disadvantageously low so that the crystal structure grows to lower the strength when a temperature raises to 400.degree. C. or more.
In order to overcome this disadvantage, an Fe-Ni alloy or molybdenum which is resistant to a high temperature having a platinum plating on the surface thereof is employed. However, other drawbacks of the occurrence of corrosion through pinholes and of cracks along bending portions may be produced.
Conventionally, a composite wire has been prepared by inserting a metal pipe to be employed as a jacket into a metal billet to be employed as a core, extruding and drawing it, or by inserting a metal pipe to be employed as a jacket into a metal rod to be employed as a core and repeating pultrusion and annealing.
In case of the extrusion, however, depending on the metal, for example, when an alloy consisting of W-Mo-Fe-Ni which has high strength at a high temperature is employed, the extrusion can be hardly carried out to perform the stable production due to the blockage of the extrusion metal.
On the other hand, in case of the pultrusion and the annealing, the sufficient bonding strength cannot be obtained because the penetration of a drawing lubricant, air and a gas.
Further, due to the difference of the workability (malleability) between the jacket and the core, the workability and the productivity are low because the peeling, the cracking and the cutting of the wire are likely to occur during the drawing process so as to provide insufficient processing and to require several thermal treatments.